Aqua is Apple's name
for the Mac OS X GUI. This includes both the appearance and the behavior
of the interface. The appearance is the first thing that grabs people's
attention. Aqua employs a sort of "translucent gel" theme for
its GUI widgets (a look seen in many previous MetaTools/ MetaCreations
applications). Here's what a button looks like:

An
Aqua button

A stated design goal of
Aqua was that (to paraphrase Steve Jobs) when you see it you want to
lick it. Aqua windows continue this theme, with gel-like proportional
scroll thumbs and colored gems for title-bar widgets (many equate them
to jelly beans). Apple has posted a
QuickTime movie here that demonstrates how the basic window controls
operate. We've got the Cliff's Notes version here:

Traffic widgets

The colored buttons are
meant to resemble a traffic light: red to close the window (stop),
yellow to minimize the window (slow), and green to zoom the window (go).

X-Mouse-like
awareness

Glyphs also appear inside the widgets when the mouse is nearby
("x" for close, "-" for minimize, and "+"
for zoom), even if the window is in the background. The Aqua mnemonics
are much more comprehensive than classic Mac OS widgets (which are all
the same shape and color, with very vague glyphs), and I suspect that
they'll be accepted quickly by Mac users once they get over the initial
shock. Unfortunately, the grouping of all three widgets on the same side
of the Aqua title bar makes the close button much more prone to
accidental clicks. The reasoning behind this placement escapes me.

A second characteristic
of the Aqua look is the use of subtle "pinstripes," much like
the striping on the surface of Apple's translucent hardware. You can see
this striping on both the active and inactive window title-bars in the
movie mentioned above. The menubar, pull-down menus, and dialog boxes also sport
stripes. Subtle features like this were not possible in the days before
true-color (32-bit) video was common.

On the other hand,
nothing we've discussed so far can't be duplicated with a second
generation display layer. Indeed, it was only a few hours after the
MacWorld San Francisco keynote that Aqua themes began appearing on the net for Windows and
Linux. But now let's take a look at the features in Aqua that take
advantage of Quartz, and find some justification for all of the hard
work Apple has been pouring into the technology.

Attentive readers may
have noticed that the front-most window in the "traffic
widgets" example above casts a transparent shadow on the window
behind it. Transparency is used extensively in Aqua. Here are a two more
examples:

Transparent dialog

Transparent menu

Using transparency in
this way is trivial with Quartz, and Aqua doesn't stop at simple, static
transparency, either. Most uses of transparency have a dynamic behavior as well.
For example, pull-down menus "fade away" when they are
de-selected (rather than disappearing instantly). But the most dynamic
Aqua interface element shown at MSWF is the "dock."